Limiting aperture for electron image devices



J. HILLIER Nov. 5, 1946.

LIMITING APERTURE FOR ELECTRON IMAGE DEVICES Filed July 16, 1943 M r N mm m w M ..E 5 .H 0 j m 7.4M E Hm m m mm 5 E m 0 a attorney Patented Nov.5,1946

LIMITING APERTURE FOR ELECTRON HWAGE DEVICES James Hillier, Cranbury, N.J assignor to Radio Corporation of America, a corporation of DelawareApplication July 16, 1943, Serial No. 495,024

9 Claims. 1 This invention relates generally to electron image apparatusand more particularly to the use of an apertured diaphragm with suchapparatus for limiting the electron irradiation to a specimen underobservation while preventing electron irradiation of the specimensupport.

The intensity of electron bombardment of an electron microscope specimenis intrinsically very high, and often is of the order of 1 to 10kilowatts per square millimeten Even if only a small portion of thisenergy is absorbed by the specimen, extremely high temperatures may bereached therein, resulting in damage or complete destruction of thespecimen. Normallyan electron microscope specimen is so thin that theenergy absorbed thereby due to electron bombardment is substantiallynegligible. However, such thin specimens must necessarily haverelatively strong means of support which necessarily will absorb a largeproportion of the electron energy of the portion of the electron beamwhich impinges upon the support. If the rate at which this energy isabsorbed by the supporting means is greater than the rate at which thesupporting means can conduct it to cooler portions of the microscope,the temperature of the support will rise to a value where the specimenwill be destroyed along the line of adhesion between it and its support.

Heretofore, this difficulty has been at least partially avoided in anumber of ways. One method comprises reducing the intensity of theirradiating electron beam. A second method comprises reducing the timeof exposure of the specimen and its support to the irradiating electronbeam. A third method includes the use of a condenser lens intermediatethe electron source and the specimen, which focuses an extremely minuteimage of the electron source only on that portion of the specimen underobservation. Destruction of the specimen is substantially avoided in thelatter instance even if a portion of the irradiating beam strikesthespecimen supporting means, since the total energy content of the beam isextremely small and the area bombarded is also extremely small. Largetemperature gradients are therefore set up Within the specimen and thespecimen support, which provide rapid conduction of heat generatedtherein to cooler portions of the microscope. While the third method issatisfactory from the standpoints of economy and compactness, it issometimes found desirable to eliminate the condenser lens normallyemployed with larger electron microscopes.

The present invention comprises a relatively simple and efficient meansof protecting an electron microscope specimen wherein a relativelylarge, heavy metallic diaphragm having an extremely minute aperturetherein is disposed immediately adjacent the specimen and intermediatethe specimen and the electron source. The diaphragm should besufiiciently large to screen effectively all portions of the specimensupport with the exception of the portions immediately adjacent thespecimen. It will be seen that this arrangement permits extremely highirradiation intensity of the specimen, while limiting the total heatgenerated in the specimen and specimen support to a minimum value.Preferably, some means of adjustment should be provided for theapertured diaphragm in order that the aperture therein may coincidesubstantially with the microscope electron beam axis.

Among the objects of the invention are to provide an improved method ofand means for irradiating electronically specimens in electron imageapparatus. Another object is to provide animproved method of and meansfor limiting the electronic irradiation of a specimen in an electronmicroscope to the area immediately adjacentto and including the specimento be observed While effectively screening other portions of thespecimen support from electronic irradiation. A further object of theinvention is to provide an improved method of and means for limiting theelectronic irradiation of a specimen and its support in an electronmicroscope wherein a relatively heavy metal diaphragm having a minuteaperture therein is interposed between the electron source and thespecimen support whereby high intensity irradiation of the specimen isprovided while the major portion of the specimen support is screenedeffectively from the electron source.

The invention will be further described by reference to the accompanyingdrawing of which the single figure is a schematic diagram of a typicalembodiment thereof.

Referring to the drawing, an electron microscope or other electron imagedevice having a frame I includes a cathode 2 which is maintained at arelatively high negative potential with respect to the grounded frame Iof the image device. The cathode 2 may be heated either directly or bymeans of a conventional heater element 3. If desired, the cathode 2 maybe surrounded by a conventional apertured Wehnelt cylinder 4. Anapertured anode 5, maintained at ground potential, is disposed adjacentthe apertured end of the Wehnelt cylinder 4, thereby providing anelectron gun which generates a beam of electrons substantially definedby the dash lines a, a.

A specimen support including a wire mesh framecomprising the Wires 6,6', 6", 6 includes an extremely thin membrane 1 of collodion, for

example, which supports a specimen 8 on the reverse side thereof. Thistype of specimen support is commonly used in electron microscopetemperatures in the wires 6, E" which would damage the supportingmembrane 1. Therefore;

a limiting apertured diaphragm 9, having an aperture Hi therein; isinterposed between the anode of the electron gun and the specimensupport. The aperture l0 should preferably be extremely minute, and thediaphragm 9 should be as close as practicable to the specimen support.The limited electron beam provided by the limiting aperture ill isindicated by the dash lines I), b. It will be seen that the electronicirradiationis substantially limited to the specimen 8 and the'portion ofthe supporting membrane 1' immediately adjacent thereto. Sincetheseelements normally are extremely thin, negligible heat'isgeneratedtherein dueto electron bombardment thereof. It should beunderstood that the irradiation field may be limited to a somewhatlargerarea which may even include the-wire supportsimmediately adjacent thespecimen, providing suflicient heat dissipation is provided to preventdamage to the supporting membrane and the specimen proper.

The limiting apertured diaphragm 9 should preferably be adjustabletransversely of the axis of the electron microscope l to provide forcentering of the aperture 18 on the electron beam axis. Any well knownmethod of centering electron microscope components may be employed, orthe adjustment of the diaphragm maybe accomplishedby means of a threadedmember ii pivoted on th'e diaphragm 9 and threaded to the side of themicroscope frame I. A control knob I2 may be provided on the threadedmember H externally of the microscope frame l, for facilitatingadjustment'of the position of the diab m 9.

"The remainder of the electron image device, or electron microscope, maybe of conventional design. It may, for example, include an electronobjective lens 13, centered on the microscope axis, at apredetermineddistance from the speciment which-will depend upon thefocal length of the lensl3." Likewise, a conventional electronprojection lens 14 may be employed to enlarge and project the imagederived from the objective lens I3 upon a fluorescent screen, or otherimage device, l5. Elie electron objective and projection lenses l3, Itmay be eitherelectrostatic or electromagnetic and may of conventionaldesign depending upon the particular optical requirements of theapparatus. 7

Y Thus the invention described comprises a simple and eificient meansoflimiting the electronic irradiation of a specimen in an electron imagedevice to the portion of the specimen support immediately adjacent toand including the portion of specimen to be observed, while ef.

fectively screening the remainder of the specimen support to preventheating thereof due to electron bombardment. I

I claim as my invention: v p I V '1. In combination, an electron imagedevice including an electron source, a specimen, a specimen support, andmeansincluding a heat conductive diaphragm having a minute aperturetherein interposed between said source and said specimen and incloseproximity to said specimen support for electronically irradiating saidspecimen and for screening said specimen support, 7

I ,2. Apparatus of the typedes'cribedain claim 1 including means foradjusting'externally of said device the position of said diaphragm toprovide coincidence of said aperture with the common 'axis of saidsource and said specimen.

3. In combination, an electron image device ineluding an electronsource, a specimen, a specimen support, and means including a heatconductive metallic diaphragm having a minute aperture thereininterposed between said source and said specimen and in close proximityto said specimen support for electronically irradiating said specimenthrough said aperture and for screening the balance of said specimensupport.

4. An electron microscope including an electron source, a specimen,aspecimen support, means including a, metallic diaphragm having a minuteaperture therein interposed between said source and said specimen and inclose proximity to said specimen support for electronically irradiatingsaid specimen through said aperture and for screening the balance ofsaid specimen support, means for forming an electronic image of theirradiated portion of said specimen, and electron lens means forproviding an enlarged image of the irradiated portion of said specimen.

5. Apparatus of the type described in claim 4 including means foradjusting externally of said microscope the position of said diaphragmto provide coincidence of said aperture with the common axis of saidsource and said specimen.

6. In an electron image device comprising an electron source and aspecimen support including I radiation of said specimen comprising alimiting aperture device interposed between said source and saidspecimen support and in close proximity tojsaid support for preventingelectron irradiation of said support.

8. In combination, an electron image device including an electronsource, a specimen support for positioning a specimen, and meansincluding a heat conductive diaphragm having a minute aperture thereininterposed between said source and said specimen support and in closeproximity to said support for electronically irradiating said specimenand for screening said specimen support.

9. An electron microscope including an electron source, a specimensupport for positioning a specimen, means including a metallic diaphragmhaving a minute aperture therein interposed between said source and saidspecimen support and in close proximity to said support forelectronically irradiating said specimen through said aperture and forscreening the balance of said specimen support, means for forming anelectronic image of the irradiated portion of said specimen, andelectron iens means for providing an enlarged image of the irradiatedportion of said specimen.

. JAMES I-IILLIER.

